Acids Type

The first type consists of only one acid, hydrofluoric, which is used to dissolve quartz. The amateur should not use it unless he has adequate facilities and a background in chemistry. Hydrofluoric acid is a close relative of hydrochloric acid, but hydrofluoric acid is the meanest member of the family. A finger can be dipped in strong hydrochloric acid without too much damage, but dip it in hydrofluoric acid, and in a few days ulcers will develop that cause deep scarring. Hydrofluoric acid dissolves quartz; it will dissolve a glass bottle. It corrodes most metals. The fumes alone are sufficient to cause skin damage. It must be used in plastic containers in a well-ventilated chamber, such as under a fume hood in a chemical laboratory. It is not an acid for kitchen or basement use.

Actually, there are few occasions when fossils must be removed from a silicate environment. Fossils found in sandstone are usually poorly preserved and hardly worth the danger of acid burn. Occasionally, well-preserved calcified fossils will occur in sandstone. Such fossils can be successfully prepared with hydrofluoric acid. One such recent occurrence was a pocket of complete blastoids found in a sandstone channel-fill in early Mississippian rocks of Montana. The small pocket yielded hundreds of blastoids complete with stems and brachioles.

Since the calcified fossils were much softer than the matrix, any mechanical preparation would have destroyed them. But excellent results were achieved by placing the specimens upside down in a wide, flat, polyethylene dish under an acid hood. Technical grade (48 percent) hydrofluoric acid was poured in to a depth of I inch to eat away the fossiliferous side of the blocks. After thirty to forty-five minutes the slabs were removed from the acid with platinum-tipped tongs held with rubber-gloved hands. The slabs were then placed under running water for a minute or so. If necessary, the specimens were etched again by further acid treatment. The fossils needed only a little soap and water and brushing to complete the cleaning.

Hydrofluoric acid has one other use. Small fossils and microfossils show internal features if they can be made translucent. This can be done by changing their chemical composition from calcium carbonate (calcite or aragonite) to calcium fluoride (fluorite). Calcium carbonate will react with dilute hydrofluoric acid to do just this chemical magic, and the fossil will become a pseudomorph of itself without losing a single dimple. It will become translucent when wet and more stable chemically as well as a bit harder. The hardness of fluorite (4 on Mohs' scale) is higher than that of calcite (3 on the scale). This technique works quite well with small brachi-opods, ostracods, foraminifera, and bryozoans.

Hydrofluoric acid is expensive as well as destructive. It should be stored where it is not apt to be spilled and where it will not cause major damage if it is spilled.